1. Field of the Invention
The present invention generally relates to an injection molding machine having a check ring disposed at the front end of a screw, and more particularly to a technique for detecting the point at which the check ring closes.
2. Description of the Related Art
In an injection molding machine having a check ring disposed at the front end of the screw, the check ring is open when the metering of resin is completed. When injection starts with the check ring being open, the resin pressure in front of the check ring is raised as the screw moves forward, so the check ring is forced to move backward and closes the flow path of the resin. The time at which the check ring closes varies with the resin pressures in front of and behind the check ring at the time when injection starts. Since resin backflow occurs from the front to the back of the check ring after injection starts and continues until the check ring closes, variations in closure timing of the check ring cause the volume of injected resin to vary and accordingly affect the quality of molded articles.
To address such variations in closure timing of the check ring, a known technique attempts to stabilize the closure timings of the check ring in every molding cycle.
Another known technique for addressing the variations in closure timing of the check ring monitors the time at which the check ring actually closes. For example, a known method detects the time point at which the check ring closes by detecting a change in resin pressure during the forward movement of the screw by means of a pressure sensor disposed within the cylinder behind the check ring (upstream of resin flow) (See Japanese Patent Applications Laid-Open No. 4-53720 and No. 4-201 225, which will be hereinafter referred to as Patent Documents 1 and 2, respectively). In another known invention, resin pressures are detected by a pressure sensor disposed within the cylinder behind the check ring and the detected resin pressures are integrated and compared with a reference value. If the integrated value exceeds the reference value, an alarm is issued so that the molded article is checked for defective molding (See Japanese Patent Application Laid-Open No. 3-114721, which will be hereinafter referred to as Patent Document 3).
Another known method detects the time point at which the check ring closes by detecting the stop of rotation of the screw being rotated during injection by the torque caused by the pressure applied to the screw flights by the resin backflow occurring during forward movement of the screw (See Japanese Patent Application Laid-Open No. 2004-216808, which will be hereinafter referred to as Patent Document 4).
The inventions described in Patent Documents 1 and 2 detect the time point at which the check ring closes by detecting a change in the resin pressures detected by the pressure sensor. The check ring is open when injection starts, so the resin pressure is continuously raised as the screw moves forward. When the check ring closes during injection, the resin pressure stops being raised and starts to be reduced. This change in resin pressure enables the time point at which the check ring closes to be detected. These inventions have drawbacks, however, because they cannot detect the time point at which the check ring closes if the resin pressure does not change because the check ring is already closed when injection starts or the check ring does not close during injection.
The invention described in Patent Document 3, which detects the time point at which the check ring closes from the integrated value of detected resin pressures, can determine whether the check ring operates normally as the check ring has been closed at the start of injection or the check ring operates normally as the check ring is closed in the midst of injection process or the check ring does not operate normally as the check ring has not been closed during an injection/hold pressure process.
In each of the inventions described in Patent Documents 1 to 3, a resin pressure sensor has to be disposed in the cylinder so as to detect the resin pressure within the cylinder and behind the check ring. Further, it is necessary for the resin pressure sensor to be disposed at a point in the rear of the position of the check ring when the screw has moved backward fully. Accordingly, parts such as resin pressure sensor and amplifier have to be additionally provided. Since the resin pressure sensor is disposed at a point in the rear of the position of the check ring when the screw has moved backward fully, the position of the check ring is, if a metering stroke is small, apart from the position where the resin pressure sensor is disposed. In such a case, it is possible that the resin pressure detected by the resin pressure sensor does not match the resin pressure immediately behind the check ring, so the state of the check ring can not always be detected with accuracy.
Unlike the inventions described in Patent Documents 1 to 3, the invention described in Patent Document 4 does not require the resin pressure sensor to be disposed within the cylinder behind the check ring, but this invention cannot detect an exact time point at which the check ring closes, because the screw receives a torque due to a pressure of the resin backflow leaking through a slight gap between the check ring and the inner wall of the cylinder even if the check ring is closed. Furthermore, the flights receive a torque due to the movement of the resin accumulated in the grooves between flights of the screw during the forward movement of the screw. The screw torque varies with the viscosity and pressure (injection pressure) of the resin and may cause the screw to rotate even when the check ring is closed.